A fundamental component of wireless communications systems is the power amplifier (PA). Such wireless communications systems include a variety of broadband and multimedia services. In such systems, the power amplifier is supported by a wireless RF transmitter. The power efficiency of the PA is an important aspect of the operational efficiency of the system as a whole. The PA typically can be thought of as having three operational ranges: the linear (amplifying) region, the saturation region, and the cut-off region. In practice, the efficiency of a power amplifier is closely related to the static bias setting choice of operation region of such power amplifier. Setting a PA in the linear (amplifying) operation region of the PA will yield higher linear output characteristics, but operating with a bias in the linear region yields very low power efficiency. Setting the bias of a PA in either the saturation region or the cut-off region will yield higher power efficiency, but the output signal quality from the PA will be degraded by distortion due to the non-linearity of the PA in those regions. Such degraded output signal quality cannot meet the requirements of most RF transmitters and receivers. Power amplifiers used in current wireless transmission systems usually are biased so that their static-points are either in their linear region or linear-to-saturation region.
The non-linear operation of such power amplifiers causes distortion due to, among other things, what are referred to as intermodulation products in the output signal. Such non-linear products are so close to the desired signal and hence the non-linear products cannot be filtered out simply by conventional filtering technologies. In order to avoid such nonlinear distortions, the conventional solutions are (i) to back off the static-point of the PA from its saturation region to its amplifying region and/or (ii) to use predistortion methods to linearize the PA's characteristics.
Traditional digital and/or analog predistortion technologies for linearizing the response of power amplifiers are typically based upon the principles of error comparison and power matching. Existing predistortion methods and circuits for wireless transmitter systems offer some help in addressing the non-linear distortion of power amplifiers that are biased for initial operation either in the saturation region or close to it. However, such existing predistortion methods are typically unable to achieve an optimized balance between linearity and DC conversion efficiency because of the PA's configuration of linear-to-saturation region.
When a power amplifier is biased for initial operation in the saturation or near-saturation region, the related intermodulation products are also amplified with signal. Therefore, the PA will exhibit nonlinear characteristics such as the amplitude and phase distortions (AM-AM and AM-PM), which will lead to the undesirable inter-modulation interferences in the neighboring frequency band. There has therefore been a need for a predistortion device and method which will accurately provide a linearized output of the PA when biased in these ranges. In addition, these has been a need for a predistortion device and method which will provide a linearized output for PA's biased for initial operation in the cut-off region.